Memory Repair is a process used in the manufacture of memory integrated circuits (DRAM or SRAM) to improve the manufacturing yield. Memory, chips are manufactured with extra rows and columns of memory cells. During testing of the memory chips (while still in the wafer form), any defects found are noted in a database. Wafers that have defective die can be repaired by severing links with a pulsed laser. Systems generally utilize wafer-handling equipment that transports semiconductor wafers to the laser process machine, and obtain the information in the form of an associated database specifying where the links should be cut and performs the requisite link ablation for each wafer.
Successive generations of DRAM exploit finer device geometry in order to pack more memory into smaller die. This manufacture of smaller devices affects the geometry of the links allocated for laser redundancy. As the devices get smaller, the links get smaller and the pitch (link-to-link spacing) shrinks as well. Smaller link geometry requires a smaller spot size from the laser in order to successfully remove selected links without affecting adjacent links, preferably with little if any compromise in throughput.
All systems focus the laser-processing beam to perform memory repair and require that the surface of the link be maintained within a small tolerance of the beam waist (focus) position with depth. When the link is in the focal plane of the lens, the focused spot will be minimum size. At focus or “beam waist height” above or below nominal, the spot will be defocused with the magnitude of defocus increasing with distance from nominal. A defocused spot reduces the energy that is delivered to the target link possibly leading to insufficient cutting of the link. A defocused spot may also place more laser energy on adjacent links or on the intervening substrate leading to possible substrate damage. At some level of defocus, the laser cutting process is no longer viable.
The allowable tolerance for relative placement of the lens and link is referred to as “depth of focus” (DOF). The depth of focus criteria is a function of the process tolerance for the particular link and laser combination. Experiments are typically performed over a range of operating parameters, including focus height, in order to determine the sensitivity of the laser cutting process to the parameters. For instance, from these experiments it might be found that the laser would reliably sever links when the combinations may exhibit more or less process latitude to focus height
Prior generation memory repair systems perform a focus operation once per site. As more dies are processed within a single site, the site dimensions get larger. This presents a problem in that the wafers seldom are flat (planar) and parallel to the focal plane. If focus is performed at only one point within a site, then the system will operate slightly out of focus at points within the site that are not near to the focus location
At least three factors affect the ability of a memory repair system to maintain the link in focus.    1. The process or sensor used to measure focus may exhibit errors.    2. The wafer may exhibit “topology” that requires different focus heights at different locations over the surface of the wafer.    3. The mechanism used to provide relative motion between the wafer surface and focal plane may exhibit errors.
A process for compensating height variations was used in 1992 by a predecessor company of the assignee of the present invention (i.e. “GSI”) to perform thin-film trimming on integrated circuits (IC) in non-wafer form. At the time, IC's were being packaged into sensors and then trimmed after packaging. The problem encountered at the time was due to the packaged die being significantly non-parallel to the surrounding package (typically pressure sensors). Incorporating a Z-Roll-Pitch mechanism for positioning the device in the product solved the problem at the time. An auto-collimator sensor was included in the optical path and used to measure the angle of the die surface relative to the focal plane. The angular information from the auto-collimator was combined with a single focus measurement to define a plane. The mechanism then moved the die in 3 axes to place the die into the best-fit plane compensating for Z, roll and pitch. The range of die tilting was sufficiently large that it was often necessary to perform iterative corrections to properly focus the die. After making an adjustment in Z, roll and pitch, a second set of focus and tilt measurements was made followed by a subsequent (smaller) focus and tilt correction.
One problem of this approach is that the auto-collimator worked best when it could be directed at a large “planar” object. With pressure sensors, it was often possible to define a large region that lacked surface features in order to use as an auto-collimator target. It would not be possible to find such a region on a typical IC found in memory repair applications.
In 1994, GSI developed a different approach to handle thin-film trimming on “tilted die.” The problem was again due to trimming on packaged IC (pressure sensors). In this case, the specifics of the customer's device precluded the use of a tilting Z-stage. A single Z-axis stage was used in the product and the Z-stage was moved in coordination with X and Y positioning of the laser beam. Also, the absence of suitable target structures for the auto-collimator on certain customer's devices forced GSI to develop the multi-site focus algorithm. Height measurements were obtained using a sensor that obtained a sequence of measurements along the z-axis from which the position of best focus was correlated to surface position—a prior art method known as “depth from focus”. The process was repeated at 3 non-collinear locations. A best-fit plane (exact in the case of 3 points) was used to coordinate the movement of the device that was mounted to the Z-stage.
Prior art laser-based, dynamic focus techniques and/or associated “depth from focus” are widely used over a range of scales and at various operating speeds. Exemplary systems operating at a microscopic scale are disclosed in U.S. Pat. Nos. 5,690,785, 4,710,908, 5,783,814, and 5,594,235, and selected pages of Chapter 7 entitled “Optics for Data Storage” in the book “Laser Beam Scanning” by Marcel Dekker, Inc., 1985. A desirable improvement in the memory processing or the processing of other microstructures would provide capability to generate and apply industry-leading small spot sizes to the applications with improved throughput. In turn, an improved figure of merit for resolution and speed in the presence of local depth variations which substantially exceed the DOF associated with the small spot sizes.